This library is not available as a prebuilt like other optional system libraries in order to encourage clients to access it via the androidx.window wrapper library. However, Chrome uses it directly in order to reduce the binary size overhead of the wrapper library. Googlers, see: b/233785784.
As with other optional SDK libraries, these classes are not compiled in. They are available at runtime through a system library loaded by the framework when a <uses-library> tag exists in an app's AndroidManifest.xml (which is added via manifest merging when depending on the GN target).
Not all devices have this optional library available. You can check if it's available via org.chromium.window.WindowApiCheck#isAvailable().
Library documentation: https://source.android.com/docs/core/display/windowmanager-extensions
Library source code: https://cs.android.com/androidx/platform/frameworks/support/+/androidx-main:window/extensions/
Run update_sources.sh